1. Field of the Invention
The present invention relates to a print head for use in an ink jet printing apparatus that performs printing by ejecting ink.
2. Description of the Related Art
A common ink jet printing scheme uses, for example, electrothermal transducing elements (heating elements) as energy generating elements for ejecting ink droplets. The ink jet printing scheme applies a voltage to each of the heating elements to instantaneously boil ink in the vicinity of the heating element. Then, the changing of the phase of the ink rapidly generates a bubbling pressure to eject the ink at a high speed.
The ink jet printing scheme allows the arrangement of heating elements having a reduced size as a result of a process similar to a semiconductor manufacturing process. This eliminates the need for a large space inside a print head. The scheme is also advantageous in that for example, the print head has a simple structure and allows arranging ejection ports densely.
The configuration of a print head of this kind will be described. The print head comprises an element substrate having heating elements allowing ink to be ejected and an orifice plate joined to the element substrate. The orifice plate has a plurality of ejection ports through which ink droplets are ejected, bubbling chambers which communicate with the ejection ports when the orifice plate is jointed to the element substrate and which serve as energy acting chambers, and ink channels that are in communication with the bubbling chambers. The combination of the ejection port, the energy acting chamber, and the ink channel is called a nozzle. Each of the heating elements is buried in that part of walls defining the internal space of the bubbling chamber which corresponds to the inside of the element substrate. The heating element is driven to generate bubbles inside the bubbling chamber so that the bubbling pressure of the bubbles causes the ink to be ejected through the ejection port. Furthermore, an ink supply port is formed in the element substrate so as to penetrate the element substrate from an obverse surface that is in contact with the orifice plate to a back surface located opposite the obverse surface.
In the print head configured as described above, the ink is fed from the ink supply port through the ink channel to the interior of the bubbling chamber, which is thus filled with the ink. The ink filled into the bubbling chamber is blown in a direction almost orthogonal to the obverse surface of the element substrate by bubbles resulting from film boiling caused by driving the energy generating element. The ink is thus ejected through the ejection port as ink droplets.
There has recently been a demand for a printing apparatus achieving printing at a high resolution. Thus, there has been a demand for a print head having finer ejection ports formed therein. However, linearly and densely arranging the ejection ports reduces the distance between the adjacent ejection ports and thus the distance between the bubbling chambers corresponding to the ejection ports. This reduces the thickness of the wall between the bubbling chambers and of the wall between the ink channels. Thus, disadvantageously, for example, the adhesion between the element substrate and the orifice plate is degraded to allow the orifice plate and the element substrate to break off easily from each other.
Thus, as described in Japanese Patent Laid-Open No. 2006-315395, two rows of ejection ports may be arranged on the same side of a common linearly extending ink supply port so that the ejection ports in one of the rows are staggered with respect to the ejection ports in the other row. This arrangement of the ejection ports ensures an appropriate distance between the adjacent bubbling chambers with the ejection ports densely arranged. This allows an increase in the thickness of the wall between the bubbling chambers, improving the adhesion between the element substrate and the orifice plate.
However, this arrangement of the ejection ports prevents the distance from the ink supply port to each of the ejection ports from being fixed. That is, some of the ejection ports on the orifice plate are located at a relatively long distance from the ink supply port, whereas the others are located at a relatively short distance from the ink supply port. This also prevents the distance from the ink supply port to each of the energy generating elements corresponding to the ejection ports from being fixed.
Thus, a variation in the distance from the ink supply port to the ejection port or the energy generating element varies the ejection characteristics of the ejected ink. An increase in the longer distance from the ink supply port to the ejection port or the energy generating element increases the speed at which the ink is ejected and the flow rate of the ink. This is because the variation in the distance from the ink supply port to the ejection port varies the resistance of the ink flow in the ink channel between the ink supply port and the ejection port. The increased length of the ink channel increases the friction between the ink and the ink channel acting until the ink is ejected. This in turn increases an inertia force required to move the ink. Consequently, the resistance offered by the ink in the ink channel during ejection increases consistently with the length of the ink channel. The increased resistance reduces the amount by which bubbles generated by heat from the heating element are expanded, when the ink is ejected through the ink supply port, in a direction opposite to that from the ink supply port to the ejection port (that is, the direction from the ejection port toward the ink supply port). Thus, a force resulting from the bubbling pressure by which the bubbles push the ink away has a reduced component traveling from the ejection port to the ink supply port. This correspondingly increases the amount by which the bubbles are expanded in an ejecting direction from the heating element toward the ejection port. This in turn increases the magnitude of an ejecting-direction component of the force resulting from the bubbling pressure. The increased magnitude of the ejecting-direction component of the force resulting from the bubbling pressure increases the flow speed and rate of the ink ejected through the ejection port.
FIG. 11 is a table showing the relationship between the distance from the ink supply port and the speed and flow rate of the ejected ink. FIG. 11 is a table showing a comparison of the speed of the ink ejected through the ejection port between an ejection port located at a longer distance from the ink supply port and an ejection port located at a short distance from the ink supply port wherein an electrothermal transducing element shaped substantially like a square 15 μm on a side is used as an electrothermal transducing element.
On the basis of the speed of the ink ejected through the ejection port located at the shorter distance from the ink supply port, the speed of the ink ejected through the ejection port located at the longer distance from the ink supply port was divided by the speed of the ink ejected through the ejection port located at the shorter distance from the ink supply port, to determine a speed ratio of 1.2. Thus, a variation in the distance from the ink supply port to the ejection port varies the speed of the ink ejected through the ejection port. The ink speed exhibited a similar trend regardless of whether the ejection amount was 0.6, 0.8, or 1.1 (pl).
When the increased distance from the ink supply port to the ejection port excessively increases the speed of the ejected ink, fine droplets are separated from the droplets, resulting in ink mist. In particular, if a large amount of ink mist occurs, the mist may adhere to and contaminate the interior of the printing apparatus. The contaminant may in turn adhere to and contaminate a print medium. Furthermore, the ink mist adhering to a sensor located in the ink jet printing apparatus may cause the apparatus to malfunction.
FIG. 12 is a graph showing the trend of the relationship between the speed of the ejected ink and the amount of ink mist generated, which relationship is observed when at most 1 pl of ink is ejected. In the graph in FIG. 12, the axis of ordinate indicates the amount of ink mist generated. The axis of abscissa indicates the speed of the ejected ink. Now, focus is placed on the amount of generated ink mist in FIG. 12. The figure then shows that once the ink speed exceeds a certain value, the amount of ink mist generated increases consistently with the ink speed.
Furthermore, if the flow rate of the ejected ink varies among the ejection ports, when the ink is placed on the print medium, the density of the resultant image may vary. The increased flow rate of the ejected ink makes the image darker, whereas the reduced flow rate of the ink makes the image lighter. The excessively increased flow rate of the ejected ink disturbs the flow of the ejected ink. Then, when the ink impacts the print medium, the shape of resultant dots may vary.
Here, to set the same ejection speed and the same ejection amount for the ejection ports arranged at the different distances from the ink supply port, it is possible to reduce the width of the ink channel to the ejection port located at the shorter distance from the ink supply port to increase flow resistance to adjust the resistance of the ink. However, the reduced ink channel width may reduce the robustness of the ink channel. With reference to FIGS. 13 and 14, description will be given of a specific example in which the reduced ink channel width reduces the robustness of the ink channel. FIG. 13 is a table and a graph showing a variation in the viscosity resistance of the ink in the ink channel caused by an error of ±1 μm in the width dimension of the ink channel with respect to a reference ink channel width of 8 or 6 μm; the error occurred during the manufacture of print heads. FIG. 14 is a table and a graph showing a variation in the inertia resistance of the ink in the ink channel caused by an error of ±1 μm in the width dimension of the ink channel with respect to a reference ink channel width of 8 or 4 μm; the error occurred during the manufacture of print heads. For description of FIGS. 13 and 14, the ejection port having a relatively large channel length from the ink supply port is defined as a long nozzle. The ejection port having a relatively small channel length from the ink supply port is defined as a short nozzle. For an ink channel width of 8 um, when a dimensional variation of ±1 μm occurs during the manufacture of the print heads, the flow resistance (viscosity resistance and inertia resistance) varies in substantially the same manner for the long nozzle and for the short nozzle. However, if the width of the ink channel to the short nozzle is reduced to set the flow resistance in the ink channel at substantially the same value for the long nozzle and for the short nozzle, the viscosity resistance and inertia resistance of the short nozzle vary more significantly when the variation of ±1 μm occurs. Thus, even a slight dimensional variation during the manufacture of the print heads significantly varies the characteristics of the ejected ink. A manufacturing process used to manufacture the print heads thus needs to be very precise, resulting in the need for much effort for the manufacture. Therefore, the reduction in ink channel width is not preferable.
To reduce the flow rate of the ink ejected through the long nozzle, the diameter of the ejection port may be reduced. However, even though this method enables a reduction in ink flow rate, it is difficult for the method to reduce the speed of the ejected ink.